Microwave phase shifters are a critical component of a transmit/receive (T/R) module in passive electronically scanned arrays (ESAs), and are used widely in commercial and other applications. Utilizing low loss phase shifters in a T/R module lowers the power requirements, and hence lowers the number of components required. This may in turn lead to smaller size and lower costs. The T/R module operating at Ku-band frequencies (e.g., between about 12 GHz and about 18 GHz) may enable the use of ESAs and ESA antennae for wide-swath, high-resolution synthetic aperture radar (SAR), imaging of terrestrial snow covers, etc. For a T/R module having four transmit channels and eight receiver channels, a 5-bit phase shifter to handle 32 signals separated by their respective phases is a useful component.
Different types of digital phase shifters have been implemented in the past using Monolithic Microwave Integrated Circuit (MMIC) and Complementary Metal-Oxide Semiconductor (CMOS) technologies. MMIC based phase shifts are often large in size, exhibit large loss, and may be subject to low yield. CMOS based phase shifters are often compact in size, but in order to compensate for the loss and noise, such phase shifters (which are active phase shifters) require a T/R module at each antenna element. This greatly increases the cost of the CMOS based phased arrays. By comparison, phased arrays for which one T/R module may be connected to multiple low-loss phase shifters affords a lower component count, and are thus is less expensive.
A phased array may be implemented using any of ferrite-based phase shifters, semiconductor-based (PIN diode or transistor) phase shifters, and MEMS-based phase shifters. Phase shifters may be implemented using several different topologies, such as switched-line, distributed MEMS transmission line (DMTL), quasi lumped element or reflect line configurations. Generally, these topologies permit for design of phase shifters up to 6-bits. Phase shifters are also capable of achieving frequency reconfiguration using liquid-crystal, photonic and/or ferroelectric technologies. Phase shifters designed using the above technologies are capable of performing a specific task over a single band of interest.
MEMS-based technology, in particular, has the ability to achieve low loss, improved matching, low direct current (DC) power consumption, and improved phase accuracy of the transmitted signals over a frequency band of interest, as compared to other contemporary solid state technologies such as PIN diodes and transistor-based switches (e.g., FET switches), while maintaining a relatively compact size. The MEMS-based phase shifters may be designed as either analog or digital. Analog phase shifters, as the name refers, may be used for controlling the insertion phase within 0-360° by means of varactors. Digital phase shifters may be used for producing discrete phase delays, which may be selected by means of switches (switched line, loaded line phase shifter) or varactors (a DMTL phase shifter). Therefore, to fulfill demand for modern RF systems and for high-precision instrumentation, it is desirable to implement a phased array using MEMS-based digital phase shifters.
However, with each of the above referenced technologies (including MEMS) for implementing phase shifters in an RF phased array, it is challenging to achieve low loss with acceptable phase shift and with acceptable repeatability within a small area. These challenges become even greater with higher bit-configurations, and with lower microwave frequency, such as frequencies below 20 GHz.
DMTL is one choice that yields relatively good insertion loss performance. However, operation of DTML becomes nonlinear with variation of phase delay over the operating frequency band once it crosses the Bragg frequency, which occurs when a unit cell of the DMTL is one third of the wavelength of the operating frequency. Moreover, area (along the length) of the DMTL phase shifter necessarily becomes large with higher-bit (e.g., 3-bit or greater) configurations at lower frequency (e.g., 20 GHz or lower).
Furthermore, in a conventional switched line 5-bit phase shifter, a minimum of 10 switches must be activated at any given time in order to achieve the desired phase shift. Stated differently, each section of the conventional switched line phase shifter controls a single bit based on the state of two switches (one switch on either side of the 1-bit section). However, it is desirable that fewer switches be activated at a given time, in order to reduce power consumption of the phase shifter.